In recent years, a light emitting element using a light emitting organic compound has been actively researched and developed as a type of light emitting element. A typical light emitting element has a layer including a light emitting organic or inorganic compound (hereinafter referred to as a “light emitting layer”) interposed between a pair of electrodes. An electron and a hole are injected and transported from the pair of electrodes to the light emitting layer by applying a voltage to the element. The light emitting organic or inorganic compound is excited by recombination of those carriers (an electron and a hole) and emits light in returning from the excited state to a ground state.
Note that an excited state of an organic compound includes a singlet excited state and a triplet excited state. Light emission in the case of the singlet excited state is referred to as fluorescence, and light emission in the case of the triplet excited state is referred to as phosphorescence.
A great advantage of such a light emitting element is that the light emitting element can be manufactured to be thin and lightweight, because it is formed of a thin film of approximately submicron to several microns in thickness. In addition, an extremely high response speed is another advantage, because time between carrier injection and light emission is microseconds or less. Further, relatively low power consumption is also another advantage, because sufficient light can be provided at a DC voltage of approximately several volts to several tens of volts. Due to these advantages, the above described light emitting element attracts attention as a next-generation flat panel display.
In such a light emitting element, a pair of electrodes and a light emitting layer are formed into films. Thus, surface emission can be easily provided by forming a large-sized element. This characteristic is hard to be provided by a light source such as an incandescent lamp or an LED (point source), or a fluorescent lamp (line source). Therefore, the above described light emitting element has a high utility value as a light source for illumination or the like.
In view of an application field thereof, it can be said that the development of a white light emitting element is one of important issues as for such a light emitting element as described above. This is because, if a white light emitting element having sufficient luminance, luminous efficiency, element life, and chromaticity is provided, a high-quality full color display can be manufactured by combining the white light emitting element with a color filter and application to a white light source for a backlight, illumination, or the like can also be expected.
Currently, not a light emitting element which emits white light having a peak in each of red, green, and blue (light's three primary colors) wavelength ranges but a light emitting element which emits white light in which complementary colors (for example, blue light emission and golden yellow light emission) are combined (hereinafter referred to as a “dual wavelength white light emitting element”) is the mainstream of the white light emitting element (for example. Reference 1: Chishio Hosokawa et al., SID' 01 DIGEST, 31.3 (pp. 522-525)). In Reference 1, white light emission is achieved by laminating two light emitting layers each emitting a complementary color so as to be in contact with each other. Such a dual wavelength light emitting element has high luminous efficiency and can have relatively favorable element life. Initial luminance of 400 cd/m2 and luminance half-life of 10000 hours are achieved in Reference 1.
The dual wavelength white light emitting element can provide favorable white color in the CIE chromaticity coordinate. However, its emission spectrum is not continuous and has only two peaks having a complementary color relationship. Thus, the dual wavelength white light emitting element is hard to provide broad and near-natural white light. When a spectrum of one of complementary colors increases or decreases depending on current density or lighting time, chromaticity tends to shift far away from white. Considering a full color display combined with a color filter, when a spectrum of one of complementary colors increases or decreases, transmission spectra of red, green, and blue color filters do not match with emission spectra of the element, and a desired color is hard to be provided.
On the other hand, not only the dual wavelength white light emitting element as described above but a white light emitting element with emission spectra having a peak in each of red, green, and blue wavelength ranges (hereinafter referred to as a “triple wavelength white light emitting element”) has also been researched and developed (for example, Reference 2: J. Kido et al., Science, Vol. 267, pp. 1332-1334 (1995), and Reference 3: J. Kido et al., Applied Physics Letters, Vol. 67 (16), pp. 2281-2283 (1995)). Reference 2 shows a structure of laminating three red, green, and blue light emitting layers, and Reference 3 shows a structure of adding red, green; and blue light emitting materials to one light emitting layer.
However, the triplet wavelength white light emitting element is inferior to the dual wavelength white light emitting element in terms of luminous efficiency and element life, and needs to be significantly improved. It is known that such an element as described in Reference 2 often does not provide stable white light; for example, a spectrum changes depending on current density.
In addition, a white light emitting element is attempted to be obtained in a different perspective from References 1 to 3 (for example, Reference 4: Japanese Patent Laid-Open No. 2003-264085, and Reference 5: Japanese Patent Laid-Open No. 2003-272860). In References 4 and 5, high current efficiency (luminance obtained with respect to certain current density) is attempted to be obtained by laminating a plurality of light emitting elements in series and overlapping light emitted from each of the light emitting elements. It is also disclosed that the white light emitting element can be provided by laminating light emitting elements which emits different colors of light in series.
However, in the case of providing, for example, a triple wavelength white light emitting element by the methods disclosed in References 4 and 5, three elements need to be laminated in series. In other words, if a white light emitting element having a spectrum in a wide wavelength range (a white light emitting element in which many different colors of light emission are mixed) is to be manufactured, the number of light emitting elements to be laminated in series increases significantly, and drive voltage multiplies. Since a plurality of light emitting elements are laminated in series, the laminated light emitting elements increase in total thickness and become susceptible to optical interference. Therefore, it becomes difficult to finely tune an emission spectrum.
As described above, a conventional dual wavelength white light emitting element has high emission efficiency and favorable element life; however, it has a problem of not having a spectrum in a wide wavelength range. Accordingly, chromaticity of white light tends to change over time. A conventional triple wavelength white light emitting element has a problem in that a spectrum shape tends to depend on current density in addition to low emission efficiency and short element life. Further, if a white light emitting element having a spectrum in a wide wavelength range is to be provided by the methods disclosed in References 4 and 5, the number of light emitting elements to be laminated in series increases significantly, and drive voltage rises considerably. Therefore, the conventional methods are not practical.